As far as I can tell a large percentage of Brexit supporters believe demonstrably untrue things about what Brexit is and its likely results, so they are supporting it based on magical thinking. This is the same issue with Trump supporters in the US.
As a nice cross country example here’s what the Economist found from some Trumpkins. They praised his:
1) honesty
2) work on lowering the national debt
3) having China pay money to our farmers
4) finally getting health care ‘done’
5) building of the wall
They are just living in a fantasy world. Similarly, it seems like U.K. conservatives think Brexit means:
1) not abiding by EU market regulations but getting to remain in the market
2) not having to abide by EU freedom of movement laws but still getting to move freely wherever they want
In both cases they have substituted what they wish were true for what is true.